Electronic structure and effective mass of pristine and Cl-doped CsPbBr_(3)

Organic–inorganic lead halide perovskites(LHPs) have attracted great interest owing to their outstanding optoelectronic properties.Typically,the underlying electronic structure would determinate the physical properties of materials.But as for now,limited studies have been done to reveal the underlying electronic structure of this material system,comparing to the huge amount of investigations on the material synthesis.The effective mass of the valance band is one of the most important physical parameters which plays a dominant role in charge transport and photovoltaic phenomena.In pristine CsPbBr_(3),the Fr?hlich polarons associated with the Pb–Br stretching modes are proposed to be responsible for the effective mass renormalization.In this regard,it would be very interesting to explore the electronic structure in doped LHPs.Here,we report high-resolution angle-resolved photoemission spectroscopy(ARPES) studies on both pristine and Cl-doped CsPbBr_(3).The experimental band dispersions are extracted from ARPES spectra along both ■ and ■ high symmetry directions.DFT calculations are performed and directly compared with the ARPES data.Our results have revealed the band structure of Cl-doped CsPbBr_(3) for the first time,which have also unveiled the effective mass renormalization in the Cl-doped CsPbBr_(3) compound.Doping dependent measurements indicate that the chlorine doping could moderately tune the renormalization strength.These results will help understand the physical properties of LHPs as a function of doping.

Combinatorial Synthesis and High-Throughput Characterization of Microstructure and Phase Transformation in Ni-Ti-Cu-V Quaternary Thin-Film Library

Ni-Ti-based shape memory alloys(SMAs)have found widespread use in the last 70 years,but improving their functional stability remains a key quest for more robust and advanced applications.Named for their ability to retain their processed shape as a result of a reversible martensitic transformation,SMAs are highly sensitive to compositional variations.Alloying with ternary and quaternary elements to finetune the lattice parameters and the thermal hysteresis of an SMA,therefore,becomes a challenge in materials exploration.Combinatorial materials science allows streamlining of the synthesis process and data management from multiple characterization techniques.In this study,a composition spread of Ni-Ti-Cu-V thin-film library was synthesized by magnetron co-sputtering on a thermally oxidized Si wafer.Composition-dependent phase transformation temperature and microstructure were investigated and determined using high-throughput wavelength dispersive spectroscopy,synchrotron X-ray diffraction,and temperature-dependent resistance measurements.Of the 177 compositions in the materials library,32 were observed to have shape memory effect,of which five had zero or near-zero thermal hysteresis.These compositions provide flexibility in the operating temperature regimes that they can be used in.A phase map for the quaternary system and correlations of functional properties are discussed w让h respect to the local microstructure and composition of the thin-film library.

Applications for Nanoscale X-ray Imaging at High Pressure

Coupling nanoscale transmission X-ray microscopy (nanoTXM) with a diamond anvil cell (DAC) has exciting potential as a powerful three-dimensional probe for non-destructive imaging at high spatial resolution of materials under extreme conditions. In this article, we discuss current developments in high-resolution X-ray imaging and its application in high-pressure nanoTXM experiments in a DAC with third-generation synchrotron X-ray sources, including technical considerations for preparing successful measurements. We then present results from a number of recent in situ high-pressure measurements investigating equations of state (EOS) in amorphous or poorly crystalline materials and in pressureinduced phase transitions and electronic changes. These results illustrate the potential this technique holds for addressing a wide range of research areas, ranging from condensed matter physics and solidstate chemistry to materials science and planetary interiors. Future directions for this exciting technique and opportunities to improve its capabilities for broader application in high-pressure science are discussed.

Revisiting the capacity-fading mechanism of P2-type sodium layered oxide cathode materials during high-voltage cycling

P2-type sodium layered oxide cathode (Na_(2/3)Ni_(1/3)Mn_(2/3)O_(2)P2-NNMO) has attracted great attention as a promising cathode material for sodium ion batteries because of its high specific capacity. However, this material suffers from a rapid capacity fade during high-voltage cycling. Several mechanisms have been proposed to explain the capacity fade, including intragranular fracture caused by the P2-O2 phase transion, surface structural change, and irreversible lattice oxygen release. Here we systematically investigated the morphological, structural, and chemical changes of P2-NNMO during high-voltage cycling using a variety of characterization techniques. It was found that the lattice distortion and crystal-plane buckling induced by the P2-O2 phase transition slowed down the Na-ion transport in the bulk and hindered the extraction of the Na ions. The sluggish kinetics was the main reason in reducing the accessible capacity while other interfacial degradation mechanisms played minor roles. Our results not only enabled a more complete understanding of the capacity-fading mechanism of P2-NNMO but also revealed the underlying correlations between lattice doping and the moderately improved cycle performance.

Revealing the inhomogeneous surface chemistry on the spherical layered oxide polycrystalline cathode particles

The hierarchical structure of the composite cathodes brings in significant chemical complexity related to the interfaces,such as cathode electrolyte interphase.These interfaces account for only a small fraction of the volume and mass,they could,however,have profound impacts on the cell-level electrochemistry.As the investigation of these interfaces becomes a crucial topic in the battery research,there is a need to properly study the surface chemistry,particularly to eliminate the biased,incomplete characterization provided by techniques that assume the homogeneous surface chemistry.Herein,we utilize nano-resolution spatially-resolved x-ray spectroscopic tools to probe the heterogeneity of the surface chemistry on LiNi0.8Mn0.1Co0.1O2 layered cathode secondary particles.Informed by the nano-resolution mapping of the Ni valance state,which serves as a measurement of the local surface chemistry,we construct a conceptual model to elucidate the electrochemical consequence of the inhomogeneous local impedance over the particle surface.Going beyond the implication in battery science,our work highlights a balance between the high-resolution probing the local chemistry and the statistical representativeness,which is particularly vital in the study of the highly complex material systems.

Oxygen vacancy-rich amorphous FeNi hydroxide nanoclusters as an efficient electrocatalyst for water oxidation

In this work,a one-pot strategy is presented to directly synthesize amorphous Fe_(x)Ni_(y) hydroxide nanoclusters(denoted as ANC-Fe_(x)Ni_(y),<2 nm)with oxygen vacancies induced by ionic liquids.The ANC-Fe_(x)Ni_(y) catalyst presents abundant catalytic sites and high intrinsic conductivity.As such,the optimized ANC-Fe_(1)Ni_(2) exhibits high activity in oxygen evolution reaction(OER)with a Tafel slope of 39 m V dec^(–1) and an overpotential of 266 m V at 10 m A cm^(-2).Notably,the optimized ANC-Fe_(1)Ni_(2) shows an extraordinarily large mass activity of 3028 Ag_(FeNi)^(–1) at the overpotential of 300 m V,which is~24-fold of commercial RuO_(2) catalyst.The superior activity of these Fe_(x)Ni_(y) hydroxide nanoclusters is ascribed to(i)the amorphous and distorted structure with abundant oxygen vacancies,and(ii)enhanced active site density by downsizing the ANC-FexNiyclusters.This strategy provides a novel route for enhancing OER electrocatalytic performance and highly encouraging for the future application of amorphous metal hydroxides in catalysis.

Mesoscale interplay among composition heterogeneity,lattice deformation,and redox stratification in single-crystalline layered oxide cathode

Single-crystalline layered oxide materials for lithium-ion batteries are featured by their excellent capacity retention over their polycrystalline counterparts,making them sought-after cathode candidates.Their capacity degradation,however,becomes more severe under high-voltage cycling,hindering many high-energy applications.It has long been speculated that the interplay among composition heterogeneity,lattice deformation,and redox stratification could be a driving force for the performance decay.The underlying mechanism,however,is not well-understood.In this study,we use X-ray microscopy to systematically examine single-crystalline NMC particles at the mesoscale.This technique allows us to capture detailed signals of diffraction,spectroscopy,and fluorescence,offering spatially resolved multimodal insights.Focusing on early high-voltage charging cycles,we uncover heterogeneities in valence states and lattice structures that are inherent rather than caused by electrochemical abuse.These heterogeneities are closely associated with compositional variations within individual particles.Our findings provide useful insights for refining material synthesis and processing for enhanced battery longevity and efficiency.

Data-Driven Lithium-Ion Battery Cathode Research with State-of-the-Art Synchrotron X-ray Techniques

CONSPECTUS:The lithium-ion battery(LIB)is a tremendously successful technology for energy storage thanks to its favorable characteristics including high energy density,long lifespan,affordability,and safety.It has been widely adopted in sectors including consumer electronics and electric vehicles,which are featured by an enormous market value.To meet the ever-increasing demands for energy density and cycle life,industry and academia are continuously devoting efforts to improve the current LIB technology.This requires an in-depth understanding of the electrochemical reaction processes and degradation/failure mech-anisms,to which advanced characterization is pivotal.Combining advanced synchrotron X-ray techniques with machine learning(ML)methods has been demonstrated as a powerful tool for uncovering the fundamental reaction and aging mechanisms in LIB and is emerging as an important research frontier.Our group’s research has been focusing on the battery cathode,which is a major limiting factor in today’s LIB technology.The degradation and failure of cathode materials in LIB are multiscale.The chemo mechanical processes at these different length scales are intertwined and mutually modulated.Therefore,it is crucial to understand the underlying mechanisms of charge−lattice−morphology−kinetics interactions in battery cathodes as a function of the electrochemical states.Synchrotron X-ray technology has unique advantages.It can detect lattice structure,electronic structure,chemical valence state,and multiscale morphology in different experimental modes,with high resolution and high efficiency.However,the large-scale experimental data bring great challenges in terms of reduction,analysis,and interpretation.Data-driven methods based on ML can greatly assist researchers to understand,control,and predict the electrochemical behavior of the complex battery cathode systems.In this Account,we focus on showcasing the integration of synchrotron and ML techniques for LIB cathode research.We review our recent findings on charge−lattice−morphology−kinetics in LIB cathode materials via this approach.First,the ML-based morphological study of cathode materials is discussed,highlighting a ML-assisted automatic feature recognition,particle identification,and statistical analysis of the prolonged cycling-induced particle damage and detachment from the carbon matrix.Second,we discuss the chemical heterogeneity and lattice deformation in cathode materials revealed by ML-assisted multimodal synchrotron characterizations.The role of ML tools in identifying and understanding chemical outliers and lattice defects in NCM cathodes is highlighted.Third,we provide our perspective on a future“dream”experiment for investigating the spatial distribution of cation−anion redox coupling effects in the battery cathode by means of resonant inelastic X-ray scattering(RIXS)imaging with ML.We anticipate that this new approach will provide new horizons for the development of novel high-energy and high-power-density LIB cathode materials.With an emphasis on the data-driven approaches for researching battery materials with synchrotron X-ray techniques,we hope that this Account will lead to more endeavors in this research field.

Itinerant to relocalized transition of felectrons in the Kondo insulator CeRu_(4)Sn_(6)

The three-dimensional electronic structure and the nature of Ce 4f electrons of the Kondo insulator CeRu_(4)Sn_(6)are investigated by angle-resolved photoemission spectroscopy,utilizing tunable photon energies.Our results reveal(i)the three-dimensional k-space nature of the Fermi surface,(ii)the localized-to-itinerant transition of f electrons occurs at a much high temperature than the hybridization gap opening temperature,and(iii)the“relocalization”of itinerant f-electrons below 25 K,which could be the precursor to the establishment of magnetic order.

Biophotonic rogue waves in red blood cell suspensions

Rogue waves are ubiquitous in nature,appearing in a variety of physical systems ranging from acoustics,microwave cavities,optical fibers,and resonators to plasmas,superfluids,and Bose–Einstein condensates.Unlike nonlinear solitary waves,rogue waves are extreme events that can occur even without nonlinearity by,for example,spontaneous synchronization of waves with different spatial frequencies in a linear system.Here,we report the observation of rogue-wave-like events in human red blood cell(RBC)suspensions under weak light illumination,characterized by an abnormal L-shaped probability distribution.Such biophotonic extreme events arise mostly due to the constructive interference of Mie-scattered waves from the suspended RBCs,whose biconcave shape and mutable orientation give rise to a time-dependent random phase modulation to an incident laser beam.We numerically simulate the beam propagation through the colloidal suspensions with added disorder in both spatial and temporal domains to mimic random scattering due to Brownian motion.In addition,at high power levels,nonlinear beam self-focusing is also observed,leading to a dual-exponential probability distribution associated with the formation of multiple soliton-like spots.Such rogue wave events should also exist in environments with cells of other species such as swimming bacteria,and understanding of their underlying physics may lead to unexpected biophotonic applications.

Engineering the electronic and strained interface for high activity of PdMcore@Ptmonolayer electrocatalysts for oxygen reduction reaction

Alloyed nanoparticles with core-shell structures provide a favorable model to modulate interfacial interaction and surface structures at the atomic level,which is important for designing electrocatalysts with high activity and durability.Herein,core-shell structured Pd3M@Pt/C nanoparticles with binary PdM alloy cores(M=Fe,Ni,and Co)and a monolayer Pt shell were successfully synthesized with diverse interfaces.Among these,Pd3Fe@Pt/C exhibited the best oxygen reduction reaction catalytic performance,roughly 5.4 times more than that of the commercial Pt/C catalyst used as reference.The significantly enhanced activity is attributed to the combined effects of strain engineering,interfacial electron transfer,and improved Pt utilization.Density functional theory simulations and extended X-ray absorption fine structure analysis revealed that engineering the alloy core with moderate lattice mismatch and alloy composition(Pd3Fe)optimizes the surface oxygen adsorption energy,thereby rendering excellent electrocatalytic activity.Future researches may use this study as a guide on the construction of highly effective core-shell electrocatalysts for various energy conversions and other applications.

Oxygen-Reconstituted Active Species of Single-Atom Cu Catalysts for Oxygen Reduction Reaction

Identification of an active center of catalysts under realistic working conditions of oxygen reduction reaction(ORR)still remains a great challenge and unclear.Herein,we synthesize the Cu single atom embedded on nitrogen-doped graphene-like matrix electrocatalyst(abbreviated as SA-Cu/NG).The results show that SA-Cu/NG possesses a higher ORR capability than 20%Pt/C at alkaline solution while the inferior activity to 20%Pt/C at acidic medium.Based on the experiment and simulation calculation,we identify the atomic structure of Cu-N_(2)C_(2) in SA-Cu/NG and for the first time unravels that the oxygenreconstituted Cu-N_(2)C_(2)-O structure is really the active species of alkaline ORR,while the oxygen reconstitution does not happen at acidic medium.The finding of oxygen-reconstituted active species of SA-Cu/NG at alkaline media successfully unveils the bottleneck puzzle of why the performance of ORR catalysts at alkaline solution is better than that at acidic media,which provides new physical insight into the development of new ORR catalysts.